UK Defence Secretary Sir Michael Fallon has refused to discuss or provide details of the alleged Trident II D5 ballistic missile test launch failure.

The unarmed missile was test-fired from the Royal Navy Vanguard-class submarine HMS Vengeance in June 2016.

It has been claimed the projectile veered off course of its original sea target in Africa during the exercise, instead heading in the opposite direction towards the US, reported the Sunday Times.

Michael Fallon was quoted by The Telegraph as saying: “Contrary to reports in the weekend press, HMS Vengeance and her crew were successfully tested and certified as ready to rejoin the operational cycle.

"We do not comment of the detail of submarine operations."

He added: "The capability and effectiveness of the UK's independent nuclear deterrent is not in doubt.

"The government has absolute confidence in our deterrent and in the Royal Navy crews who protect us."

Meanwhile, an unnamed US defence official was reported by CNN as saying that the Trident II D5 missile deviated from its intended path as part of an automatic self-destruct sequence, which is triggered when a missile detect an irregularity.

Fallon refused to comment on CNN's claims, and was quoted by media sources as saying: “We do not give operational details of the demonstration and shake-down operation of one of our submarines conducting a test with one of our Trident missiles.”

British Prime Minister Theresa May also repeatedly declined to answer directly when questioned about the misfire during a BBC interview.

May’s spokeswoman said that the prime minister was briefed shortly after taking office last year that a nuclear submarine and its crew had been successfully certified in tests.

"Contrary to reports in the weekend press, HMS Vengeance and her crew were successfully tested."

Declining to discuss operational details of tests, the spokeswoman was quoted by Reuters as saying: "On taking office, the current prime minister was briefed on a range of nuclear issues, including this."

The UK Parliament voted in favour of the Trident renewal programme in July 2016, agreeing to replace the current fleet of four Vanguard-class nuclear warhead-carrying submarines with four Dreadnought-class vessels.

The upgrade is projected to cost £40bn, and is scheduled to become operational by 2028.

Image: The UK Royal Navy’s Vanguard-class submarine HMS Vengeance, which is armed with the Trident ballistic missile. Photo: courtesy of POA(Phot) Tam McDonald / MoD.